Experiments with many systems indicate that local intercellular interactions are important in the processes of determination, morphogenesis, and differentiation. Although the cell surface is involved, there is very little known about the basic cellular or molecular events underlying intercellular interactions during early development. As many congenital abnormalities involve breakdowns in the cellular interactions that regulate developmental processes, an understanding of the mechanisms of these interactions will further our understanding of developmental problems. The embryonic cells of Drosophila (including the imaginal discs of the larva) represent one of the best characterized and most tractable systems for examining these interactions in early development. Initially, we will make monoclonal antibodies against imaginal discs and embryos. The antibodies will be screened by immunofluorescence against a variety of larval tissues, and the fluorescence distributions will be compared to other information concerning the determination and development of the undifferentiated cells to select antibodies of interest. These antibodies will be used in more extensive immunofluorescence analyses - both to characterize the antibodies and to learn more about various aspects of Drosophila development. The antigens defined by the antibodies will be characterized biochemically, using immunoabsorbant techniques for their isolation. Two approaches will be used to test hypotheses concerning the functions of the antigens; we will attempt to specifically block developmental processes using antibodies and we will examine the effects of eliminating the antigens at various times and places during development, using genetic techniques. Finally, the biochemical and functional analyses will be combined to define functional domains within the antigen molecules. In summary, we will examine the surface antigens of undifferentiated, essentially embryonic cells. Our approach will allow us to define and characterize molecules involved in early developmental processes. Using the wide variety of techniques available with Drosophila, it should eventually be possible to define, in molecular terms, how these molecules function and how their expression is controlled.